1. Field of the Invention
This invention relates generally to a portable cell balancing repair tool for correcting cell-to-cell voltage imbalances within a battery pack and, more particularly, to an automatic and portable cell balancing repair tool for correcting cell-to-cell voltage imbalances within a battery pack that charges individual cells to a common voltage set-point.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium ion, nickel metal hydride, lead acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules including several battery cells to meet the vehicle power and energy requirements. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
When a battery pack for a vehicle is being manufactured, individual battery cells are typically combined as groups of cells, which may include three cells, and groups of cells are welded together to form a battery module. The battery modules are electrically coupled together within a housing to provide the battery pack where the number of modules determines the battery pack voltage. During the manufacturing process, battery packs and battery cell sections are electrically charged and discharged to identify battery pack build and cell issues and failures. These failure modes found at the electrical test of the battery pack can cause individual cells or cell groups to become out of balance. A cell out of balance condition is where cells within the battery pack or section are at different states-of-charge (SOC). There are various reasons why the individual cells or cell groups may be out of balance. For example, a defective voltage temperature sense monitoring (VTSM) harness may cause poor connections to the battery pack. This may result in poor cell-to-cell voltage balancing where if the cell voltage imbalance is large enough, the performance of the battery pack is affected, where the drive distance of the vehicle is reduced at the battery beginning of life. Cells are considered to be too out of balance with each other when the section or battery pack open circuit voltage range is greater than the battery build process specification for a particular battery configuration.
When a battery pack is tested and the charge of a cell or cell group is identified as being too low, which causes a cell-to-cell voltage imbalance, then affirmative steps need to be taken to correct the imbalance. There are known methods for raw battery cell-to-cell voltage balancing in a manufacturing repair environment using a power supply and a resistor to manually repair a battery section or module that is out of balance. However, these known methods of manually repairing a battery section or module have typically been burdensome, ineffective and unsafe. What typically happens is that if a low charge cell is found in a cell group, then the entire module that is welded together is discarded and replaced, which has obvious cost implications because most of the cells in the module may be operating properly. Currently, there is no automatic battery balancing repair tool that can be purchased off the shelf.